1. Technical Field
The present invention relates to components used in broadband spectrum analyzers. More particularly the present invention relates to circuitry for extending the frequency range of a spectrum analyzer into the millimeter-wave spectrum.
2. Related Art
Currently available broadband spectrum analyzer architectures make use of switched filters along the RF signal path as a means of implementing frequency-band pre-selection. FIG. 1 provides circuitry illustrating the use of switched filters to allow frequency-band pre-selection in a broadband spectrum analyzer. The circuitry includes a low-frequency spectrum analyzer (LFSA) 2. The LFSA 2 receives RF and IF signal inputs. In low-frequency mode, the RF signal input is connected directly to the LFSA 2 by means of switch 7. In high-frequency mode, switches 6 and 7 are connected to one of bandpass filters 4 to provide an RF signal to mixer 8. The IF output of mixer 8 is then provided as a second RF input to LFSA 2.
As the frequency range of the architectures using switched filters illustrated in FIG. 1 extends into the millimeter-wave spectrum, frequency-dependent RF-switch limitations occur. Switching limitations such as increased loss and reduced bandwidth begin to take their toll on spectrum analyzer performance. The direct results of these limitations are a decrease in the dynamic range of the instrument, and severe boundaries on its operating frequency bandwidth.
In order to overcome the later limitation, techniques using external mixers have been developed to extend the frequency range of the spectrum analyzer. FIG. 2 illustrates such an external mixer configuration, also known as an external frequency-range extension. The circuitry includes a LFSA 2 that receives an RF signal directly in a low-frequency mode, and receives the RF signal through an external mixer 12 in a high-frequency mode. In low-frequency mode, the RF signal input is connected directly or by means of switch 9 to a low frequency RF input of the LFSA 2. In high-frequency mode, the switch 9 directs the RF signal through a frequency range extender 10 that includes mixer 12. The mixer 12 output provides a second RF input to the LFSA 2. The LO input signal to the mixer 12 is generated inside the LFSA 2.
One drawback to using the technique illustrated in FIG. 2 is that measurement must typically be performed manually within separate high-frequency bands each requiring an external frequency range extender. Manual measurements are necessary due to the fact that typically the extension mixer 12 covers only a portion of the high-frequency band and must be replaced with different extension mixers for covering other bands. In addition, the lack of filtering on the IF port of the external mixers adds to the tediousness of the measurements.
It would be desirable to extend the operating frequency range of a LFSA without the drawbacks and the limitations of the frequency dependent RF-switch of FIG. 1, or the need to perform measurements manually within separate frequency bands as when an external mixer of FIG. 2 is used.